chore: update header-map module references to use docplanner namespace CORE-384

[rodrigoechaide]

Summary

• Rename module namespace: Update the Go module path and all Traefik plugin references from rodrigoechaide/header-map to docplanner/header-map across go.mod, .traefik.yml, docker-compose.yml, traefik.yaml, and README.md.
• Upgrade golangci-lint config: Migrate .golangci.yml to v2 format, enable default: all with a refined disable list, and add formatters (gci, gofmt, gofumpt, goimports).
• Fix all lint issues (26 total):
  • revive/var-naming: Rename Go package from header_map to headermap (no underscores).
  • err113: Replace dynamic fmt.Errorf calls with wrapped static sentinel errors (errEmptyMappings, errMissingSourceHeader, errMissingTargetHeader).
  • canonicalheader: Use canonical HTTP header names in test assertions (X-App-Id, X-Ua-Device).
  • paralleltest: Add t.Parallel() to all test functions and table-driven subtests.
  • wsl_v5: Add required blank lines before if statements and assignments following different statement types.
• Update LICENSE copyright from personal name to Docplanner.
• Fix README: Correct Docker port references (8888 / 9090) to match docker-compose.yml.

Test plan

• make lint passes with 0 issues
• go test ./... passes (all tests use t.Parallel() and canonical headers)
• Verify Traefik local dev setup still works with the renamed plugin path (docker compose up)

[ishioni]

If we have ci config, we should probably do some workflows to run that CI...